Bodies of chromium boride and chromium molybdenum and their production



I parts.

BODIES OF CHROMIUM BORIDE AND CHROMIUM MOLYBDENUM AND THEIR PRODUCTIQN Frank W.'Glaser, Bronx, N. Y., assignor to Borolite Corporation, Pittsburgh, Pa., a corporation of Delaware No Drawing. Application April 20, 1955, Serial No. 502,740

Claims. (Cl. 29-1825) This invention relates to chromium-boride containing structural materials or compositions of matter which exhibit high hot strength, high heat shock resistance and high corrosion resistance at elevated temperatures, and to the production of such materials.

Among the objects of the invention are structural materials for use in applications such as critical gas turbine parts and the like, which require a material which exhibits high hot rupture and impact strength, high heat shock resistance and high resistance to corrosion under adverse conditions at which other known materials have only a short life or are otherwise unsatisfactory.

Chromium has long been known as a material which has high corrosion resistance at high temperatures, this corrosion resistance being secured by the formation of a corrosion resistant chromium oxide surface stratum on the chromium surface which is exposed to oxidizing com- ,has only relatively low creep resistance at high temperatures and for this reason, chromium cannot be used by itself in applications which require low creep resistance at high temperatures.

The best prior efforts to combine chromium with boron for its low creep resistance at elevated temperatures are described in Cole et al Patent 2,088,838, the material of this patent having become known as Colmonoy. This Colmonoy material contains as principal ingredients three chromium borides, CrBz, CrB and CI3B2, and the conglomerate of these three chromium borides, as described in the Colmonoy patent may also contain in free, compounded or alloyed state, minor further additions of chromium, boron, aluminum and iron.

However, the Colmonoy material of the type described in Cole et al. Patent No. 2,088,838has excessive brittleness, and could not be utilized for producing, either by itself or by cementing particles thereof with any of the known cementing additions, a cemented material that would exhibit the required high strength, and that would have the capacity of elastically yielding under a load at elevated temperatures as required in certain gas turbine My U. S. application Serial No. 363,972, filed June 22, 1953, discloses the discovery of a new material combining chromium with dichromium boride CrzB which has many desirable characteristics making it much superior to previously known cemented chromium boride materials including the above referred to Colmonoy material, the material of the said invention being substantially free of the boron-rich chromium boride compounds such as CrBz, CrB, and CraB. In view of the holdings of the Patent Ofi'ice that the expression free of (a certain substance) constitutes a negative limitation and should not be used in the claims of this application there is used the expression that the material of the invention contains at most a negligibly minute amount or at most 0.01% of 2,807,076 Patented Sept. 24, 1957 a specified substance and such expression shall be construed to mean that the material of the invention is substantially free of such specified substance.

The present invention is based on the discovery of a new material much superior in rupture strength to those disclosed in my said application Ser. No. 363,972 and having all its other desirable properties, which is obtained by combining chromium diboride CrzB with a chromiummolybdenum alloy with the ratio of the diboride to the alloy therein varying from 95/5 to 10/90 and the ratio of the chromium to the molybdenum of the alloy varying from 95/5 to 50/50.

For best results, the material of the invention should be free of carbon impurities greater than about 0.1% and free of iron impurities greater than about 0.15%, and nitrogen content greater than about 0.05%. (Throughout the specification and claims, all proportions are given by weight unless otherwise specifically stated.)

One phase of the invention is based on the discovery that the temperature at which the strong material of the invention may be produced by sintering or hot pressing may be controllably varied by controlling the oxygen impurities of the material. Thus, if the unmolded material is made under conditions which leave therein a maximum of 1.5% of O2 and 0.3% N, the desired shaped material may be produced by sintering between 1470 C. and 1530 C. On the other hand, if the unmolded material is made under conditions which reduce the oxygen impurities to at most 0.1% and the nitrogen impurities to at most 0.05%, the desired shape material may be produced by sintering at 1610 C. Such change in the sintering temperature also controls the strength of the resulting material with the higher rupture strength being obtained with the material which has been sintered at the higher temperature. Thus, a sintered material of the invention consisting of 20% CrzB and Cr-Mo alloy consisting of 80% Cr and 20 Mo had a stress-to-rupture life of 100 hours under 14,000 p. s. i. (pounds per square inch) at 760 C. when produced with oxygen impurities of 1% by sintering at 1470 C. Similar material produced with oxygen impurities reduced to at most 0.1% and sintering at 1680 C. had stress-to-rupture life of 200 hours under the same load and temperature conditrons.

The strong, hard material of the present invention which combines chromium diboride CrzB and a chromium molybdenum alloy may be produced by first compacting the diboride particles and the alloy powder particles to produce a green compact and thereafter sintering at the elevated temperature to form out of the green compact the desired hard material. It also may be produced by hot pressing the mixture of the ingredients in a suitable die such as a graphite die.

The dichromium boride material CrzB may be produced by direct synthesis of chromium and boron having a purity of at least about For best results, the impurities should not exceed about 2.5% and improved results are obtained when the impurities are only about 0.5% or less. The commercially available electrolytic chromium and commercially available amorphous boron of such purity may be used for producing this material. Instead of amorphous boron, crystalline boron may be used. In producing the dichromium boride, chromium and boron powders are mixed in proportions correspnding to the stoichiometric proportions of CrBz, and the mixed powder ingredients are subjected to a heat treatment in which small amount of boron powder present combines with the chromium powder to form dichromium boride CrzB.

Satisfactory results are obtained with-mixing the rea satisfactory alloying.

proportions of the chromium powderand the boron powder which have been comminuted to a particle hours does notresult in an improved final material.

The ball-milled mixture of the properly proportioned powder ingredients is then heated in a protective atmosphere :within a crucible at 'a temperature of 1300' to l350 C. or in generalbetween 12001500 C. until the amorphous boron has been purified and the reaction between the chromium and boron has reached equilibrium condition. Good results are obtained with a heat treatment of from one to two hours Which yields the body of dichromium boride CrzB.

The amorphous boron contains magnesium oxide as a major impurity ingredient, and the heattreatment at 1300 C. in a hydrogen atmosphere within a graphite crucible reduces the magne sium oxide to magnesium, and the resulting magnesium volatilizes at 1300 C. leaving in the crucible thepurified boron which forms the desired CrzB as the equilibrium conditions are reached in the progress of the heat treatment.

In an alternative procedure, the dichromium boride CraB may be obtained by mixing electrolytic chromium and purified boron in the desired proportions, in which case the initial comminution of the individual powders by a gaseous vortex pulverizing mill is not required. This procedure requires boron of the highest possible purity of not less than about 95% of its total, as chemically analyzed. The properly proportioned mixture of the electrolytic chromium and pure boron powder ingredients is then ball-milled to mix, and then subjected to a similar heat treatment in a graphite crucible within a hydrogen atmosphere at 1300 C. to 1350 C. until the reaction between the chromium and the boron present in the mixture yields at equilibrium condition the dichromium boride Cr'zB. t

' .The chromium molybdenum alloy maybe prepared .by mixing the desired proportions of chromium and molybdenum powders and tumbling for 24 hours in a steel null to insure a good mixing, introducing into a j vitrified crucible, tamping in the crucible and sintering heating at an alloying temperature between 1300 and 1600 C. in a suitable protective atmosphere until the components have been thoroughly alloyed followed by cooling and comminution to the desired particle size such :as minus 100 mesh. Good results are. obtained with e lectroly tic chromium powder of minus 325 mesh particle size having 99 purity with a maximum iron content of 03% and molybdenum powder of 99% purity and minus 100 mesh particle size. The mixture of the desired proportions of chromium-molybdenum alloy such as 80 chromium and 20% molybdenum is charged in through a .suitable crucible such as a vitrified crucible of aluminum oxide '(Alundum) or a zirconium crucible which may be held enclosed in a graphite crucible. The interior ofthe crucible'is purged 'by passing therethrough an inner gas such as purified helium or argon. 'Good results are=obtained by argon which is passed through a titanium sponge purification vessel containing titanium sponge heated to 950 C. After purging the crucible with the argon the heat is turned on andafter heating at 150.0 C. for one hour the heat is turned off and the contents cooled. Heating for 24 hours at 1430 C. also produces The so-treated' sintered body of chromium-and. molybdenum powder is then crushed and neduced to alloy. powder of. minus 100 mesh par- "Av. coef. V of thermal ticle'size. The sintered mass may first be broken into chunks by a 'drop hammer and then milled as by a steel coffee mill and finally reduced to size by balling for 24 hours with steel balls. -An analysis of a sample of the alloy powder produced by the procedure just described shows that at least 97.7% of its content consisted of 79% Cr and 20.8% 'Mo and that'it. contained less than 0.3% iron and less than 0.1% carbon. X-ray analysis of the powder showed .a single Cr-Mo phase without additional lines.

7 Example 20% of CrzB, prepared as disclosed above and 80% of the chromium'molybdenum (SO/'20) alloy-prepared as described above are provided. The dichromium boride CraB has been comminuted to a particle size of 4-6 microns and the chromium molybdenum alloy 'has a particle size of mesh. These ingredients are mixed and ground together in a dry state in a steel ball mill for 24 hours. Thereafter, 1% of a phenolic binding. resin (such as a'phenol formaldehyde condensation product f I in the B state) is added and'mixing is continued for another'hour. This resultant powder mixture is then cold pressed in a-steel die lubricant with a stearic 'acid acetone solution-at 1 0-12 t. s.. i. (tons persquare inch). The

resin binder of the green compact is then cured at 230 C. for /2 hour in. an air circulating oven. This produces the green compact which maybe ground tosize and to remove imperfections if desired.

The green compact is then placed 'in oxide (Alundum) boat covered with Alundum granules covered with a'molybjdenum shield and heated in a furnace in a dry hydrogen atmosphere. The temperature of the dry hydrogen atmosphere is maintained at 1400 C., raised to 1550" C. for an instant'and then slowly redried to 1400 C. as the-boat is slowly removed from the furnace into the cooling zone The sintering treatment takes /2 hour or longer. The resultant sintered body has a'density of 97%. a

The following properties have been 'found'for this product.

Temp'era- Lite Load ture, F. (hours) NACA heat shock Over 100 cycles.

I test Transverse rupture V (1000 C.) 150,000 p. s. i. NACA drop test 3.7"/1bs. Specific heat (1000 V V I V a C 1287 cal./;grm./ C.

expansion 7 100 hr ./1000 C., O mg.[cm.

Oxidation resistance As indicated above, there is a wide range/within which The following, table illustrates the proportions. and

properties of various compositions which may bemade as described above.

an aluminum Approx. stress rup- Trans- Approx. ture strength at verse Exam- GraB/ melt. 1,800 F. rupture ple (Jr-Mo Or/Mo p oiont, strentgth 100 hr. life 100 hr. 1,800 F.

(p. s. 1.) life The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific features or details shown and described in connection with the exemplifications thereof.

I claim:

1. A hard body of .high strength, consisting essentially of dichromium boride and a chromium molybdenum alloy, the dichromium boride comprising 10 to of said body, the balance of said body consisting essentially of a chromium molybdenum alloy containing 60% to 95 chromium.

2. A hard body of high strength, consisting essentially of dichromium boride and a chromium molybdenum alloy, the chromium molybdenum alloy comprising 60-95% of chromium, substantially the entire remainder being molybdenum.

3. A hard body of high strength, consisting essentially of dichromium boride and a chromium molybdenum alloy, the dichromium boride comprising 10 to 95% of said body, the chromium molybdenum alloy comprising 60-95% of chromium, substantially the entire remainder being molybdenum.

4. In the process of making a high temperature resistant chromium-boron-molybdenum article, the steps comprising providing 10-95% of dichromium boride powder containing at most negligible minute amounts of other chromium borides, providing 590% of a chromium molybdenum alloy powder consisting essentially of chromium and molybdenum in proportions between 95/5 and 50/50, thoroughly mixing said powder components, adding a resinous binder to the components, pressing said powder to provide a green compact bonded by said resin and sintering the green compact at around 1400-1550 C. to provide a bonded article.

5. In the process of making a high temperature resistant chromium-boron-molybdenum article, the steps comprising providing 10-95 of dichromium boride pow der containing at most negligible minute amounts of other chromium borides, providing 590% of a chromium molybdenum alloy powder consisting essentially of chromium and molybdenum in proportions between 95/5 and 50/50, thoroughly mixing said powder components, pressing said powder to provide a green compact and sintering the green compact at around 1400-1550 C. to provide a bonded article.

No references cited. 

1. A HARD BODY OF HIGH STRENGTH, CONSISTING ESSENTIAL OF DICHROMIUM BORIDE AND A CHROMIUM MOLYBDENUM ALLOY, THE DICHROMIUM BORIDE COMPRISING 10 TO 95% OF SAID BODY, THE BALANCE OF SAID BODY CONSISTING ESSENTIALLY OF A CHROMIUM MOLYBDENUM ALLOY CONTAINING 60% TO 95% CHROMIUM.
 4. IN THE PROCESS OF MAKING A HIGH TEMPERATURE RESISTANT CHROMIUM-BORON-MOLYBDENUM ARTICLE, THE STEPS COMPRISING PROVIDING 10-95% OF DICHROMIUM BORIDE POWDER CONTAINING AT MOST NEGLIGIBLE MINUTE AMOUNTS OF OTHER CHROMIUM BORIDES, PROVIDING 5-90% OF A CHROMIUM MOLYBDENUM ALLOY POWDER CONSISTING ESSENTIALLY OF CHROMIUM AND MOLYBDENUM IN PROPORTIONS BETWEEN 95/5 AND 50/50, THOROUGHLY MIXING SAID POWDER COMPONENTS, ADDING A RESINOUS BINDER TO THE COMPONENTS, PRESSING SAID POWDER TO PROVIDE A GREEN COMPACT BONDED BY SAID RESIN AND SINTERING THE GREEN COMPACT AT AROUND 1400*-1550* C. TO PROVIDE A BONDED ARTICLE. 